Blurred or smeared images occur when there is relative motion between objects in the image scene and the image sensor (e.g., film). This occurs when light from objects in the scene to be imaged is transposed over many pixels of the image sensor. This is referred to as pixel smearing. A common example of pixel smearing is an image of a fast moving object or scene taken with an average or long exposure time. Alternatively, pixel smearing can occur when the object is stationary, but the camera is moving at a fast rate of velocity. This a common problem in aerial surveying photography where images are taken from a moving platform such as a plane, a satellite, or the international space station (ISS).
In an example of the ISS, a camera on board of the ISS is moving at about 7.4 km/sec with respect to the Earth. In low light conditions, the exposure time can be one second or longer. During the one second exposure, given that a single pixel of an image sensor projects to approximately 70 meters on the ground, each pixel will integrate light (photons) from a 7.4 km long strip. Understandably, this causes intense pixel smearing and blurred images.
One way to reduce smear is to apply forward motion compensation (FMC). This approach has long been applied to film based aerial surveying cameras where the film is moved through the focal or image plane at a rate such that a spot on the film is always seeing the same spot on the ground. However, the film-based FMC approach has several shortfalls. First, it only compensates for one degree of motion. Secondly, the use of a shutter is required to stop the film exposure. This limits the ability to make a number of consecutive image frames. Finally, film is limited in spectral sensitivity and the resultant images are difficult to process and transmit in real time from an orbiting platform.
Another conventional approach to FMC is to use electronic sensors such as charge coupled device (CCD) sensors. One such approach uses a time delay integration (TDI) CCD sensor where photoelectric charges generated in row of pixels (perpendicular to the direction of motion) are electronically transferred to a readout integrated circuit at a rate such that the signal from a discrete position on the ground continues to be integrated during an exposure. The TDI-CCD approach is intended primarily to compensate for low signal conditions in pushbroom imaging. However, TDI-CCD sensors also have several shortfalls. First, it does not address the distortion effects inherent in pushbroom imaging systems which use wide-angle lenses. Further, to compensate for motion during a long exposure (e.g., one second or longer), a continuous 2D push broom image cannot be obtained as many pixels (e.g., hundreds) in a row are integrated to generate a single signal. Finally, CCD sensors are restricted to a certain spectrum and are not sensitive to infrared wavelengths. Accordingly, an improved FMC system is needed for an infrared-based imaging system.